The latest development sources can be obtained as follows:
(assuming you have a copy of tla; if not, install the tla package of your OS or distribution (if available); otherwise obtain the latest releases from ftp.gnu.org and build on your machine)
Once you have "tla" in your path, and you are checking out the tla sources for the the first time, do
cd your_work_directory
tla register-archive http://www.atai.org/archarchives/lord@emf.net--2005-MIRROR/
tla register-archive http://www.atai.org/archarchives/atai@atai.org--public/
tla get tla--atai-dists--1.3.4 tlasrc
cd tlasrc
tla build-config config
Then, afterwards, you can
cd tlasrc/tla
tla update
to sync your copy with the latest sources. The official sources are signed with this PGP public key
Or use the daily snapshots or the release candidates (when near the release of the new version)